1. Field of the Invention
The present invention relates to the control of the combustion of an internal combustion engine and particularly to a method of estimating the intensity of knocking of the engine with a sensor placed in the engine.
2. Description of the Prior Art
The engine comprises at least one cylinder comprising a combustion chamber within each cylinder having an internal lateral wall, pistons with a top which slides in the cylinders and a cylinder-head. Generally, a carbureted mixture is enclosed in the combustion chamber and undergoes a step of compression followed by a step of combustion under the effect of controlled ignition using a spark plug. These steps are grouped together under the term “combustion phase” in the subsequent description.
In a combustion engine, the burning of the air/gasoline mixture normally begins after the spark. The flame front propagates and its blast thrusts a part of the mixture against the walls of the cylinder and the top of the piston. The rise in pressure and in temperature becomes so great that the fuel ramming against the walls attains its self-ignition point and bursts into flame in several places. This phenomenon is called “knocking”. Thus knocking, is above all, a phenomenon of abnormal combustion in controlled-ignition engines and is perceptible externally as a metallic noise coming from the engine. It results from the occurrence of shock waves in the combustion chamber.
The micro explosions which result therefrom produce vibrations in the acoustic domain (on the order of 5 to 20 KHz). They are very vigorous and can rapidly create hot spots which will accentuate the problem still more. The accumulation of micro explosions will cause a small amount of metal to be torn away or to melt on the top of the piston and/or on the walls of the cylinder and rings. After some time (depending on the intensity) this will lead to the destruction of the piston, the rings or the walls of the cylinder.
The estimation of the intensity of knocking allows controlling of the combustion, limiting the effect of the knocking and not damaging the cylinder.
Methods of estimating the knocking of an internal combustion engine are known. They are based on the recording of signals arising from a cylinder pressure sensor. A first method applies off-line, that is to say during subsequent processing, a technique of Fourier transformations of the recorded signals to estimate the knocking. Such a method is described in the following documents:    “Application of the Reallocated Smoothed Pseudo Wigner-Ville Distribution to Knock Detection”, Olivier BOUBAL and Jacques OKSMAN, Traitement du Signal volume 15, 1998    “Knock Measurement for Fuel Evaluation in Spark Ignition Engines”, C. Hudson, X. Gao, R. Stone, Fuel volume 80 (2001)
A second method applies a technique of filtering and of detecting a maximum of the signals to estimate the knocking off-line. Such a method is described in the following document:    “Modeling of In-cylinder Pressure Oscillations under Knocking Conditions: Introduction to Pressure Envelope Curve”, G. Brecq and O. Le Corre, SAE 2005
Finally, another method is known, also based on the recording of signals arising from a cylinder pressure sensor. This time, a wavelet technique is applied to the signals to estimate the knocking off-line. Such a method is described in the following document:    “Knocking Detection Using Wavelet Instantaneous Correlation Method”, Z. Zhang and, E. Tomita, Journal of SAE Review volume 23 (2002)
However, these methods do not allow estimation of the intensity of the knocking in real time (the computation must typically be done in less than 50 μs in order to be performed onboard by a computer).
French Patent application 2,949,511 discloses a method of estimating in real time the intensity of knocking, with a vibratory signal. According to this method, the coefficients of the Fourier decomposition of this signal are determined in real time and the energy contained in the signal is determined therefrom by summing the squares of these coefficients of the Fourier decomposition. Finally, the intensity of knocking is estimated by determining a parameter correlated with the intensity of knocking equal to the square root of the maximum of the energy.